Injection molding—the process of injecting a quantity, or shot, of molten plastic or resin into a mold—is today one of the world's dominant forms of plastic article manufacture. In this process, resin is melted, mixed, and made flowable by a combination of the application of heat and kinetic energy, usually through the use of a flighted screw that rotates and reciprocates within a heated containment barrel. A mold is designed and manufactured with a cavity configuration that allows for the specific part or parts to be molded. The mold is mounted in a mold clamp and is in fluid communication with the containment barrel.
After the rotating screw and applied heat have plasticized the resin, the screw is moved forward to inject the shot of plastic from the accumulation volume into the mold. The operation known as pack-and-hold (exerting additional pressure on the resin in the mold) is accomplished by further forward movement of the screw.
Before the advent of the heated screw, the plastic or polymer simply was contained in a cylinder, which was heated by conduction. When the polymer was melted it was conveyed into an injection cylinder where a piston moved forward to inject the plastic into the mold. The piston performs the pack-and-hold operation.
The plasticizing action of the screw dramatically increased the speed of the process. Nevertheless, use of the rotating and reciprocating screw imposes several limitations on the process. First a non-return valve is included on the end of the screw to permit plastic to flow past the screw during plasticizing but not during injection. Accurate shot size is a function of the accuracy of the non-return valve. Also, the size of the shot is determined by the inner diameter of the barrel and the stroke length of the screw; thus, the rate of recovery (backward movement of the screw to pre-injection position and filling of the accumulation volume downstream of the screw) is a function of the shot size. Screw diameter should be determined only by the recovery requirements and the cylinder diameter by the force required to fill the mold. Shot size thus is a screw and barrel length design feature, and variation in the shot size without corresponding change of screw and barrel length can cause reductions in efficiency of the plasticizing process. Sometimes shut-off valves are used to halt flow between barrel and mold, and are required if the screw is to recover when the mold is open.
There are also known injection molding apparatus that employ the screw/heated barrel arrangement (without the ability to reciprocate) to fill an accumulator, which in turn provides pressure to inject shots into the mold. The accumulator also performs the pack-and-hold functions. Nevertheless, the screw must provide constant temperature and pressure to the accumulator or shot density and weight will vary. Furthermore, due to the relatively large volume of the accumulator and associated tubing and valving that are unswept by a piston, color chances are problematic, requiring lengthy purging operations and increasing the likelihood of scrap parts. The three-way valve required for this type of machine directs the flow from the screw to the accumulator and from the accumulator to the mold has been a common source of problems. If the valve fails to operate properly, high injection pressures can be directed to and exerted upon the screw, destroying the thrust assembly of the screw. Short (small volume) shots and leakage are also common problems.
A need exists, therefore, for an injection molding apparatus that avoids the drawbacks of both reciprocating screw and accumulator systems, while providing the advantages and benefits of both.